This invention relates to positive temperature coefficient (PTC) thermistor elements and heating devices using such thermistor elements. In particular, this invention relates to improvements in their heating efficiency.
PTC thermistor elements are frequently used as a heat generator for a heating device, as shown, for example, in FIGS. 9 and 10. FIG. 9 shows a prior art PTC thermistor element 1 of a type comprising a main body 2 of a PTC material and a pair of electrodes (the first electrode 3 and the second electrode 4 ) formed on its mutually opposite main surfaces. A heating plate 5 to be heated thereby may be disposed, as shown in FIG. 9, so as to contact the first electrode 3, although it goes without saying that there may be situations where a heating plate can be disposed so as to contact both of the electrodes 3 and 4. FIG. 10 shows another prior art PTC thermistor element 6 of a type having a pair of comb-shaped electrodes 8 and 9 formed on one of main surfaces of a main body 7 made of a PTC thermistor material so as to interdigitally sandwich each other. A heating plate 10 to be heated thereby may be disposed so as to contact both of the comb-shaped electrodes 8 and 9.
Problems with such prior art thermistor elements 1 and 6 are explained next. With the PTC thermistor element 1 of the type shown in FIG. 9, heat escapes through the surfaces of the main body 2 and hence its surface temperature becomes lower than the temperature at the center. The temperature difference thus generated is shown in FIG. 12. As a result, the resistance of the main body 2 becomes higher at the center and hence the electric field intensity becomes higher there, while the field intensity becomes relatively weaker in the surface regions, as shown in FIG. 11. This causes an uneven heat distribution, the center part emitting more heat, giving rise to problems in heating efficiency and thermal response regarding the heating plate 5 to be heated thereby.
The standard thickness of the thermistor main body 2 is 2 mm or more, and this means that there is a distance of greater than about 1 mm between the center of heat production and the heating plate 5 to be heated. Since PTC thermistor materials are generally a poor thermal conductor, the temperature of the thermistor main body 2 remains higher near the center in the direction of its thickness and this has the effect of limiting the current which can flow inside. In summary, the heat generated by the thermistor main body 2 cannot be efficiently propagated to the heating plate 5 to be heated.
Attempts at preventing such lowering of heating efficiency have included increasing, as much as possible, the area of contact between the PTC thermistor element 1 and the heating plate 5, but this means that the overall size of the thermistor element 1 must necessarily be increased. If the overall size of the thermistor element 1 is increased, the heating device using the thermistor element 1 becomes correspondingly larger, and this is not a desirable consequence.
The lowering of heating efficiency can be reduced also by reducing the thickness of the thermistor main body 2 but the thermistor element 1 as a whole must generally satisfy an official requirement as to its thickness. Besides, this method cannot be adopted indiscriminately because the resistance against applied voltage should not be unduly compromised.
In a PTC thermistor element 6 of the type shown in FIG. 10, heat is generated mainly around the surface area where the electrodes 8 and 9 are formed, and hence the center of heat generation can be brought closer to the heating plate 10. In other words, heat can be more efficiently propagated to the heating plate 10 than by the thermistor element 1 of the type having electrodes on two mutually opposite main surfaces. On the main surface of the thermistor element 6 facing the heating plate 10, however, it is only the area where neither of the electrodes 8 and 9 is formed that can emit heat because the areas on which the electrodes 8 and 9 are formed do not emit heat. In general, the heat-emitting portion of the main surface facing the heating plate 10 is only from 1/2 to 2/3 of the main surface area. Moreover, since the electrodes 8 and 9 protrude outward from the main surface towards the heating plate 10, there is a space created between the heat-generating portion of the main surface and the heating plate 10. Such a space serves as a thermal resistance, adversely affecting the heating efficiency.